The defaults are determined based on object size and can go up to 30 for
objects smaller than 256 bytes.  Before the previous patch changed the
accounting, this could have made cpu partial list contain up to 30
pages.  After that patch, only up to 2 pages with default allocation
order.

Very short lists limit the usefulness of the whole concept of cpu
partial lists, so this patch aims at a more reasonable default under the
new accounting.  The defaults are quadrupled, except for object size >=
PAGE_SIZE where it's doubled.  This makes the lists grow up to 10 pages
in practice.

A quick test of booting a kernel under virtme with 4GB RAM and 8 vcpus
shows the following slab memory usage after boot:

Before previous patch (using page->pobjects):
  Slab:              36732 kB
  SReclaimable:      14836 kB
  SUnreclaim:        21896 kB

After previous patch (using page->pages):
  Slab:              34720 kB
  SReclaimable:      13716 kB
  SUnreclaim:        21004 kB

After this patch (using page->pages, higher defaults):
  Slab:              35252 kB
  SReclaimable:      13944 kB
  SUnreclaim:        21308 kB

In the same setup, I also ran 5 times:

    hackbench -l 16000 -g 16

Differences in time were in the noise, we can compare slub stats as
given by slabinfo -r skbuff_head_cache (the other cache heavily used by
hackbench, kmalloc-cg-512 looks similar).  Negligible stats left out for
brevity.

Before previous patch (using page->pobjects):

  Objects: 1408, Memory Total:  401408 Used :  304128

  Slab Perf Counter       Alloc     Free %Al %Fr
  --------------------------------------------------
  Fastpath             469952498  5946606  91   1
  Slowpath             42053573 506059465   8  98
  Page Alloc              41093    41044   0   0
  Add partial                18 21229327   0   4
  Remove partial       20039522    36051   3   0
  Cpu partial list      4686640 24767229   0   4
  RemoteObj/SlabFrozen       16 124027841   0  24
  Total                512006071 512006071
  Flushes       18

  Slab Deactivation             Occurrences %
  -------------------------------------------------
  Slab empty                       4993    0%
  Deactivation bypass           24767229   99%
  Refilled from foreign frees   21972674   88%

After previous patch (using page->pages):

  Objects: 480, Memory Total:  131072 Used :  103680

  Slab Perf Counter       Alloc     Free %Al %Fr
  --------------------------------------------------
  Fastpath             473016294  5405653  92   1
  Slowpath             38989777 506600418   7  98
  Page Alloc              32717    32701   0   0
  Add partial                 3 22749164   0   4
  Remove partial       11371127    32474   2   0
  Cpu partial list     11686226 23090059   2   4
  RemoteObj/SlabFrozen        2 67541803   0  13
  Total                512006071 512006071
  Flushes        3

  Slab Deactivation             Occurrences %
  -------------------------------------------------
  Slab empty                        227    0%
  Deactivation bypass           23090059   99%
  Refilled from foreign frees   27585695  119%

After this patch (using page->pages, higher defaults):

  Objects: 896, Memory Total:  229376 Used :  193536

  Slab Perf Counter       Alloc     Free %Al %Fr
  --------------------------------------------------
  Fastpath             473799295  4980278  92   0
  Slowpath             38206776 507025793   7  99
  Page Alloc              32295    32267   0   0
  Add partial                11 23291143   0   4
  Remove partial        5815764    31278   1   0
  Cpu partial list     18119280 23967320   3   4
  RemoteObj/SlabFrozen       10 76974794   0  15
  Total                512006071 512006071
  Flushes       11

  Slab Deactivation             Occurrences %
  -------------------------------------------------
  Slab empty                        989    0%
  Deactivation bypass           23967320   99%
  Refilled from foreign frees   32358473  135%

As expected, memory usage dropped significantly with change of
accounting, increasing the defaults increased it, but not as much.  The
number of page allocation/frees dropped significantly with the new
accounting, but didn't increase with the higher defaults.
Interestingly, the number of fasthpath allocations increased, as well as
allocations from the cpu partial list, even though it's shorter.

Link: https://lkml.kernel.org/r/20211012134651.11258-2-vbabka@suse.czSigned-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>